The symptoms you describe suggest a lack of RAM. As FS
will need to keep reading and writing to and from the disk to get the info it needs. And I can tell you this disk swapping as its called takes one hell of a long time, compared to using whats in RAM.
This isn't the only reason it will run slow though. But until you give your system specs we cant diagnose the problem.
Eg, a graphics card with a lot of RAM would help as the textures can be kept in there. Also what type of CPU do you have? If its a Celeron or Duron then because these CPU's have reduced cache (memory on the chip) a huge bottleneck can occur going between the RAM and CPU.
Cant you tell I do Computer Engineering LOL